New Lower Bounds for Seven Classical Ramsey
Numbers $R(3,q)$
Kang Wu 1, Wenlong Su 2, Haipeng Luo 3, and Xiaodong Xu 3
1 South China Normal University, Guangdong 510631, P. R. China
2 Guangxi University Wuzhou Branch, Wuzhou 543002, P. R. China
3 Guangxi Academy of Sciences, Nanning 530022, P. R. China
Abstract Full Text PPT
As recorded in S. P. Radiszowski, Small Ramsey numbers, Electronic.
J. Comb., DS1 10,(2006),1-48, known results on the Ramsey numbers R(3;q) are
$R(3,25)\geq143, R(3,26)\geq150, R(3,28)\geq164, R(3,29)\geq174$.
New lower bounds for seven classical Ramsey numbers
are obtained by considering some circulant graphs $G_{n}(A_{i})$
with $n\geq91$ whose orders might be either prime or not. The
results are
Theorem. $R(3,24)\geq143,R(3,25)\geq153,R(3,26)\geq159,R(3,27)\geq167,R(3,28)\geq172,R(3,29)\geq182,R(3,30)\geq187$.
There are three formulas in S. P. Radiszowski, Small Ramsey numbers, Electronic. J. Combin., DS1 10,(2006),1-48.
R(3,4k+1)\geq6R(3,k+1)-5 (1)
R(5,k)\geq4R(3,k-1)-3 (2)
R(3,k,l+1)\geq4R(k,l)-3 (3)
As a consequence of Theorem and the formulas (1) (2) (3), we obtain
Corollary 1.
R(3,93)\geq853, R(3,97)\geq913, R(3,101)\geq949,
R(3,105)\geq997, R(3,109)\geq1027, R(3,113)\geq1087, R(3,117)\geq1117.
Corollary 2.
R(5,25)\geq569, R(5,26)\geq609,
R(5,27)\geq633,
R(5,28)\geq665, R(5,29)\geq685, R(5,30)\geq725, R(5,31)\geq745.
Corollary 3.
R(3,3,25)\geq569, R(3,3,26)\geq609, R(3,3,27)\geq633,
R(3,3,28)\geq665, R(3,3,29)\geq685, R(3,3,30)\geq725, R(3,3,31)\geq745.
The above analysis enables us to develop the
following algorithm.
Algorithm
1)Given integer $n\geq5$, let $m=[\frac{n}{2}]$. Given a 2-partition
$S=S_{1}\bigcup S_{2}$ of $S=[1,m]$ where both $S_{1}$ and $S_{2}$
are nonempty. Let $q_{i}=|S_{i}|$ for $i=1,\,2$. Let $i=1$.
2)Set $A_{i}=\{x|x\in Z_{n},x\in S_{i}=\ \text {or}\ n-x
\in S_{i}\}$. Sort $A_{i}$ lexicographically. Assume that
$A_{i}=\{x_{1},x_{2},\ldots\}$. Set $[A_{i}]=1$, $j=1$.
3)For $x_{j}\in S_{i}$, find
d_{i}(x_{j})=\{y|y\in A_{i}, y>x_{j}-y\in A_{i}\}.
If $|d_{i}(x_{j})|=0$ go to 5).
4)Find the $A_{i}$-colored chain starting with $x_{j}\in
S_{i}$. If $l_{i}(x_{j})\geq[A_{i}]$, let $[A_{i}]=l_{i}(x_{j})+1$
and print out this chain.
5)Increase $j$ by 1. If $j< q_{i}$, go to 3).
6)Let $k_i=[A_i]+1$. Increase $i$ by 1. If $i=2$, go to 2).
7)Print out $R(k_{1}+1,k_{2}+1)\geq n+1$ and the algorithm is terminated.
Given $n$ and the set $S_{1}$, the algorithm gives
the clique number $[A_{2}]$ of $G_{n}(A_{2})$ and the first clique
of length $[A_{i}]$.
We also points out other lower bounds such as
R(3,17)\geq92, R(3,18)\geq98, R(3,19)\geq106,
R(3,20)\geq109, R(3,21)\geq122, R(3,22)\geq125, R(3,23)\geq136.
These results were obtained by Wang Qingxian and Wang Gongben in
1994 and recorded in S. P. Radiszowski, Small Ramsey numbers,
Electronic J. Combin., DS1 10,(2004),1-48. According to the
reference in it, their paper [WWY1] has not been published. Hence
our work verifies these results.